World's first underwater nuclear explosion

Operation Crossroads was a series of nuclear weapon tests conducted by
the United States at Bikini Atoll in mid-1946. It was the first test of a
nuclear weapon after the Trinity nuclear test in July 1945, and the
first detonation of any nuclear device following the Fat Man detonation
on August 9, 1945. Its purpose was to investigate the effect of nuclear
weapons on naval ships.

Aerial view of the Able mushroom cloud rising from the lagoon with the Bikini Island visible in the background. The cloud carried the radioactive contaminants into the stratosphere. Operation Crossroads "Baker" shot, 1946, Bikini Atoll. (Image courtesy of US Govt. Defense Threat Reduction Agency.)

Aerial view of the Able mushroom cloud rising from the lagoon with the Bikini Island visible in the background. The cloud carried the radioactive contaminants into the stratosphere. Operation Crossroads "Baker" shot, 1946, Bikini Atoll. (Image courtesy of US Govt. Defense Threat Reduction Agency.)